Recent sampling off the eastern and south-eastern coasts of Australia has produced a number of specimens of platybranchiate sphaeromatid isopods, but the unfortunate lack of detailed keys has led to a number of misidentifications and the curation of unidentified material in museum collections. In an effort to prevent this, all known Australian platybranch species are redescribed and refigured here. The geographical ranges of Paracassidina pectinata Baker and Syncassidina aestuaria Baker are extended from Western Australia to south-eastern Australia and Queensland. The general biology of P. pectinata is discussed and adults of S. aestuaria are described for the first time. In addition, a new species, forming a new genus, Platysphaera membranata gen. nov., sp. nov., is described from the Coral Sea. A key for the identification of all known Australian platybranchs is provided.